The present invention relates to the field of electrolysis cells or installations. FIG. 1 is a diagram showing an electrolysis cells or installation 100 used for producing fluorine. The installation 100 comprises a vessel 101 containing an electrolyte 102, e.g. a solution of hydrofluoric acid (HF), and having immersed therein electrodes of two types, namely cathodes 103 and anodes 104. The anodes 104 are fastened and electrically connected to opposite sides of a busbar 105. The busbar 105 simultaneously provides support and delivers electrolysis current for the anodes 104. In well-known manner, the busbar 105 is connected to the positive terminal of a direct current (DC) generator (not shown in the figure), while the cathodes 103 are connected to the negative terminal of the generator. The anodes 104 are distributed longitudinally on either side of the busbar 105 and project beyond the bottom face 105a of the busbar.
FIG. 1 shows the electrolysis installation 100 while it is in operation, i.e. while the electrodes 103 and 104 are immersed in the electrolyte and are being fed with direct current by the generator. For example, with an electrolyte that is constituted by hydrofluoric acid, electrolysis gives rise to bubbles of gaseous fluorine 108 being given off at the anodes 104 and to bubbles of hydrogen 109 being given off at the cathodes 103. The bubbles of these two gaseous species rise to the surface of the electrolyte and are collected by independent ducts 110 and 111 in the top portion of the electrolysis installation 100. A diaphragm 112 is located level with the top portions of the electrodes so as to enable the bubbles of gaseous fluorine 108 to be collected selectively by the duct 110, and the bubbles of hydrogen 109 to be collected by the ducts 111.
The cathodes 104 and the anodes 103 are spaced apart from one another by a determined distance d in order to prevent the hydrogen and the fluorine that are given off in gaseous form from mixing. This distance d makes it possible to ensure that bubbles of gaseous fluorine 108 rise along the anodes 104 without risk of coming into contact with the bubbles of hydrogen 109 that rise along the cathodes 103.
Nevertheless, such spacing between the anodes and the cathodes reduces the efficiency of the electrolysis installation. The electrochemical efficiency or Faraday efficiency of an electrolysis installation, i.e. the ratio of the volume of gas actually produced (here fluorine) divided by the volume of gas as calculated from the electrical energy supplied, depends in particular on the distance between the anodes and the cathodes. In other words, the Faraday efficiency decreases with increasing distance between the anodes and the cathodes. Furthermore, as shown in FIG. 1, the distance d that is imposed between the various rows of electrodes 103 and 104 limits the number of rows of electrodes that can be placed inside the vessel 101, thereby penalizing the productivity of the electrolysis installation.
In order to reduce the distance the cathodes and the anodes, it is known to place a membrane therebetween. Nevertheless, in an electrolysis installation for producing gaseous species that are corrosive, such as fluorine, it is necessary for a membrane to be available that presents very particular corrosion-resistance characteristics. When producing fluorine by electrolysis, as described above, the bubbles of gaseous fluorine that are given off give rise to corrosion and to erosion of the elements of the installation with which they come into contact during electrolysis. A membrane located close to the anodes comes into contact with most of the bubbles of fluorine rising towards the surface of the electrolyte. The membrane is thus subjected to the combined corrosion-erosion phenomenon that results from the chemical nature of the gaseous species constituting the bubbles in association with the effects of the bubbles traveling along the membrane. The membrane used must also be sufficiently stiff not to move under the effect of movement of the electrolyte bath or of the bubbles, in order to guarantee good separation between the species that are given off and avoid any contact with the electrodes.
The materials presently used for making separation membranes do not enable sufficient resistance to the corrosion-erosion phenomenon to be obtained and, by degrading prematurely, they run the risk of becoming permeable to the gaseous species that are given off. One solution for providing better resistance would be to increase membrane thickness, but that would be equivalent to increasing the distance between the cathodes and the anodes, and would consequently reduce the Faraday efficiency and the productivity of the installation.